The present invention relates to a digital controlled, or pulse controlled, constant speed brushless motor actuated by a predetermined frequency and rotated continuously in accordance with the frequency with less speed irregularity, and more particularly to the motor of the type described which is used particularly for, though not limited to, direct-driving a capstan of a tape recorder, chart feed machanism for direct-writing recorders, drum drive mechanism for facsimiles, and disc rotation mechanism for floppy disc drives.
One of the typical motors of the type described is a so-called DC motor, which can be manufactured to be relatively small in size and therefore less power consumption is required. The DC motor, however, is not suitable for slow speed rotation requirement and has disadvantages that a certain controlling device is additionally required since it does not have a characteristic of constant rotational speed. In other words, the DC motor, if it should be operated at a constant speed, needs a sensor or detector of the rotational speed so as first to detect the speed difference from the predetermined speed and then to add a suitable torque which is proportional to the speed difference. Otherwise, the DC motor should be provided with another feed-back mechanism similar to the detector described. In addition to the above, in case that the DC motor is of brushless type, a rotational position sensor such as Hall element is required. Thus, the DC motor requires some special circuit devices for detecting the rotational position and the speed of the rotor thereof, the special circuit devices being generally complex and expensive, and moreover quite difficult to locate the elements in accurate position. In addition to the above disadvantage, the DC motor has another disadvantage that a very complex operating windings should be provided in the axial direction on multi-layered or laminated silicon steel plates which have numerous slots, resulting in an excessive height or axial length of the motor. Further, the height of the motor becomes greater if the aforementioned sensor or detector is provided.
A stepper motor is well known to have a step-angle advancement for use in a numerical control system. The stepper motor is synonymous with a pulse motor or a stepping motor, in which an electromagnetic force is generated for driving the rotor in certain definite angular movement when pulse signals are transmitted to a stator winding. In the stepper motor, therefore, rotational frequency of the rotor is proportional to the pulse frequency if the latter is given continuously. It is also known, however, that the stepper motor is not advantageously used as a motor for a smooth rotation as is required in the direct-driving mechanism for a capstan of a tape recorder since it produces rotational irregularity due to the step-angle advancement.